Well my question is how were you planning to fill the tank? Run it down to the service station for free air? Hmm, now that would be an idea for free power
Course lots of places now use those crappy pay machines, some are still large compressors with free air.
Anyway I had a thought on this for aircompressors in a workshop moving volumes of air.
Use about 4 large storage tanks, a motor between each one and vavle to controll the flow so that there is a large difference in presure between full and when it alough more air to enter.
First the main compresser tank builds to 120psi shuts off, turn back on at 80 PSi like normal. Using about a 1/2" pipe connect to air motor then out to next tank. do this about 4 times. Now when you start up the system the first tank pumps to about 90-100psi, valve pops on line and dumps that pressure and volume through the first motor to fill the second tank, when the second tank hits about 100psi it pops the valve to dump through the next motor and into the next tank. By the time the system is full you have dumped high volume at high pressure through about 4 motors filling 4 tanks. You should lose nothing if you have no leaks. You end up with all the compresses air filling 4 tanks at 120psi each. Now run tools!
When the first tank drops to 80psi a valve in the inlet line pops open, this aloughs the pressure to flow from the previous tank to the last tank. Being the supply tank is at 120psi it's oulet valve is already opened, you now have 40psi rushing though the motor to get to the lower pressure tank till it begins increasing in pressure. This will work backwards all the way to the main compressors tank. You should eventually have all motors running at one time. When the main tank drops to 80psi the compressor starts as normal and all motors should be running till the tanks are all full again at 120psi or so.
It's still going to take more power to pump all that air than you will get back out of it, but as it travels tank to tank like that through the motors you are getting someting back for basically nothing, if it works! Your pumping that air to run sanders and saws and drills and grinders in the first place and that is where the air pressure is doing in the end, the work it was actually compressed for to begin with. Your just storing 400gal in a bunch of tanks instead of 60gal in one tank
As for the torques of the motor, some of those air drills and die grinders seem to have pretty good power to them, how well or long they would work like this I don't know. Problem is most such tool type motors just blow out the exhaust air everywhere and not actaully a place to attache a line for that output.
A person could build an airpowered engine from a steam engine design, like for the old models, the ones that did work with lower pressure steam, maybe 45-60psi. As lsong as the first tank has more pressure than the second tanks the motor between them should run as air will flow though it while the tanks try to even out the difference or the tool is keeping air moving out the last tank as it's filling.
I think that little tank mentioned is far to small to do much, but there are some small steam engines it could run, double acting piston type with flywheel comes to mind. Air enters one end and pushes piston to other end air in that end flows out the port, flywheel slides valve plate, that end now gets air and pushes piston back to first side letting the air out the port there. This type piston engine uses the same port for both intake and exhaust, controlled by a slide plate and box covering and un covering ports to direct the flow in or out each side at the same time. Pretty simple really, I am building one (very slowly).
Should work at very low pressure but not sure the work it could do but the air would last longer. It would do more work at higher pressure but you lose your air faster. So it would be a trade off finding the sweet spot to do the most work and the longest run time on limited air supply. If nothing else it would look cool